The glutathione S-transferases are a group of enzymes that have been shown to be active in the detoxification of certain drugs, carcinogens and metabolites. Increased expression of the placental form of the enzyme has been found in association with carcinogenesis and drug resistance. Some investigators have found increased expression of the enzyme, measured by RNA slot blot and protein assays, to be inversely related to estrogen receptor positivity in malignant breast tumors. We are prospectively studying fine needle aspirates of breast cancer patients to assess glutathione S-transferase and expression of a drug resistance marker, P-glycoprotein, before and after chemotherapy to determine if a relationship exists between levels of expression and response to therapy. Thus far, we have evaluated 35 patient aspirate samples.